RJH Scientific, Inc. seeks to develop high-efficiency x-ray filters to enhance the sensitivity and substantially reduce the radiation dose for 2-D imaging and computed tomography of contrast agents in small animals and humans. These filters will increase the efficiency of anti-cancer drug discovery and improve measurements of the early effects of cancer therapy. Other potential applications of the filters include spectral optimization for mammography and differential K-edge imaging for contrast-enhanced diagnostic radiology, enabling these procedures to be performed more safely with 5 to 10 times lower radiation dose to the small animal or patient than currently possible. The goal of the Phase I research is to demonstrate by laboratory measurements of appropriate phantoms the feasibility of making filters that can be used with existing broadband x-ray sources to obtain quantitative, low radiation dose, high spatial resolution, 2-D and 3-D images in vivo of molecular probes or tracers containing iodine, barium, or xenon. Major Phase II objectives will be to fabricate the filters, evaluate their performance for quantitative, contrast-enhanced, 2-D and 3-D imaging in vivo of small animals, and extend their range of central energies. The commercial versions of the proposed filters, improvements of an existing prototype filter, will be small and robust, relatively inexpensive to manufacture, and capable of producing narrowband x-ray beams having central energies between 15 keV and 55 keV over angular extents up to 40 degrees by 60 degrees.